board: Use CONFIG_XPL_BUILD instead of CONFIG_SPL_BUILD

Use the new symbol to refer to any 'SPL' build, including TPL and VPL

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/freescale/imx8qxp_mek/Makefile b/board/freescale/imx8qxp_mek/Makefile
index acaadcd..459ec77 100644
--- a/board/freescale/imx8qxp_mek/Makefile
+++ b/board/freescale/imx8qxp_mek/Makefile
@@ -5,4 +5,4 @@
 #
 
 obj-y += imx8qxp_mek.o
-obj-$(CONFIG_SPL_BUILD) += spl.o
+obj-$(CONFIG_XPL_BUILD) += spl.o